1. Field of the Invention
The present invention relates to a wireless communication system that utilizes an equalizer such as a Viterbi equalizer and more particularly relates to the estimation of channel characteristics for use by such an equalizer.
2. Discussion of the Related Art
Wideband CDMA, one form of direct-sequence code division multiple access (DS/CDMA) transmission system, has been accepted as one of the air interface technologies for UMTS/IMT2000 and the Third Generation Partnership Project (3GPP). Using DS/CDMA techniques can provide wireless and cellular applications the efficient use of available bandwidth, immunity to interference, and adaptability to variable traffic patterns. Because the infrastructure for the new wireless networks that will exploit DS/CDMA will take a long time to develop, second generation wireless communication systems like Global System for Mobile (GSM), United States Digital Cellular (IS-54) and CDMA Digital Cellular Standard (IS-95) continue to play significant roles in wireless communication networks.
GSM is the most popular cellular standard in second generation wireless communication systems. Approximately sixty-five percent of all cellular subscribers in the world receive their primary voice transmission or Internet access from GSM networks. This market penetration alone ensures that GSM will continue to be an important technology. GSM will eventually adopt the WCDMA standard, which provides more frequency bandwidth for voice, data and video transmission. A dual mode transceiver that incorporates both standards to provide smooth migration and backward compatibility, therefore, will be desirable. For all of these reasons, research and development continues on conventional GSM technology and GSM base and mobile stations will continue to be an important market.
GSM utilizes frequency division duplex (FDD) in combination with time division multiple access (TDMA) and frequency hopping multiple access (FHMA) schemes to provide base stations with the capability to simultaneously access and separate multiple users in both the time and frequency domain. The available forward and reverse (or uplink and downlink) frequency bands are divided into 200 KHz wide channels or slots called absolute radio frequency channel numbers (ARFCNs). Each ARFCN identifies a forward and reverse channel pair that is separated in frequency by 45 MHz and each forward channel is time shared between as many as eight subscribers (mobile stations) using TDMA. That is, each of the eight subscribers uses the same ARFCN in the frequency spectrum, while they occupy unique time slots (TS) to avoid interfering with each other.
Each GSM user transmits a burst of data during the time slot assigned to it. GSM defines five specific data burst types, for example, in GSM 05.02. Normal bursts are used for the traffic channel (TCH) and the dedicated control channel (DCCH) transmissions on both the forward and reverse link. Bursts consist of 148 bits which are transmitted at a rate of 13/48*1,000,000 bits per second together with an unused guard interval of 8.25 bits provided at the end of each burst to avoid interfering with time-adjacent users if there are timing errors. Out of the total 148 bits per time slot, two blocks of 57 information bits are located close to the beginning and end of the burst. The midamble consists of a 26-bit training sequence which is assigned by GSM networks and allows the mobile stations (e.g., handsets) or the base stations to analyze the radio propagation characteristics before demodulating and decoding the transmitted symbols. On either side of the midamble there are control bits called stealing flags used to distinguish whether the time slot contains traffic (TCH) or control (FACCH) data.
One type of receiver for the demodulation of transmitted symbols in the GSM transmission system utilizes a Viterbi equalizer. In GSM and other wireless communication systems information signals to be transmitted are linearly combined due to radio propagation, and at the receiving end, a Viterbi equalizer processes the received signal samples. The Viterbi equalizer demodulates the transmitted symbols according to a process of maximum likelihood sequence estimation. To find a best (i.e., the most probable in view of the received symbols) sequence of transmitted symbols, the Viterbi equalizer needs to estimate the channel and so derive the channel information. Channel information provides a model of the channel and allows prediction of the characteristics of different symbol sequences. The equalizer uses this information to select the most likely transition to each new state by comparing the accumulated path metrics of the two predecessor states plus the so called transition or branch metrics.
Several channel estimation techniques using training sequence data have been proposed. Training sequences are standardized for any given communication system and so provide a known symbol sequence. The simplest channel estimator is based on block correlation. An example of such a simple channel estimator is described in the article, Lopes, “Performance of Viterbi Equalizers for the GSM System,” Second IEEE National Conference on Telecommunications, pp. 61-66 (1989). That article describes a channel estimation procedure involving essentially the convolution of the received signal with the stored central N bits of the training sequence. The end result of the article's channel estimation process is the channel impulse response (CIR). The accuracy obtained in estimating the channel impulse response through block correlation can be limited since the technique uses a short correlation length and so may be susceptible to additive noise sources. The article, Khayrallah, et al., “Improved Channel Estimation with Side Information,” 47th IEEE Conference on Vehicular Technology, Vol. 2, pp. 1049-53 (1997), describes both a least square (LS) and a constrained least square based channel estimator that better rejects additive noise and so improves the quality of CIR estimation. The described channel estimation processes use matrix inversion, which may require significant computation resources and might have problems due to the word length available in practical implementations.